Much progress in the electronics industry comes from circuit size reduction. This is most directly accomplished by running photolithographic processes at ever-shorter wavelengths of light. Processes using 193 nanometer (nm) light are undergoing commercialization while 157 nm wavelength light is under development as a next generation candidate.
In immersion photolithography (Switkes et al, J. Vac. Sci. Technol. B, 19 (6), 2353 6, November/December 2001) an optical source and a target surface are immersed in a highly transparent high refractive index liquid. As shown by Switkes et al Microlithography World, May 2003, pp. 4ff, higher resolution in photolithography can be achieved at a given wavelength of incident light when a high refractive index transmission medium is employed. Realization of the potential benefits of this technology is dependent upon identifying high refractive index liquids having high transparency in the VUV/DUV and excellent photochemical stability.
All known organic materials absorb to some extent UV radiation of 193 nm. The issue is whether liquids can be found that are sufficiently transparent to be practical. Short chain suitable polycyclic fluoroalkanes H(CH2)nH and short chain fluorocarbons F(CF2)nF are relatively transparent compared to their longer-chain homologues at 193 nm, as disclosed, for example, in B. A. Lombos et al, Chemical Physics Letters, 1, 42 (1967); G. Belanger et al, Chemical Physics Letters, 3(8), 649(1969); and K. Seki et al, Phys. Scripta, 41, 167(1990).
It is well known in the art that while the addition of fluorine to hydrocarbons generally improves transparency at UV wavelengths, it is usually accompanied by a decrease in refractive index. Particularly for application in the new field of immersion lithography at VUV wavelengths of 193 nm, the refractive index of most hydrofluorocarbons is much too low. Currently, water is the preferred immersion fluid, with good transparency and a refractive index of 1.44. It is desirable to identify highly transparent liquids with refractive index above 1.5.
Gaseous materials may be highly transparent, but can have lower than desired refractive indices. Liquids with low boiling points similarly can have an undesirably low refractive index and require a pressure vessel for containment.